Home Pitched Roofing Controlling the risk of condensation in loft spaces

Controlling the risk of condensation in loft spaces

by Jennie Ward

In his latest consultant case study, Technical Roofing Consultant John Mercer tackles the causes of condensation in loft spaces and explains how to treat and prevent the problem.

John Mercer, Technical Roofing Consultant.

During the winter months I often receive enquiries regarding condensation problems in loft spaces, and this year has been no exception. But it’s important to point out that often, excessive condensation in the roof space will be a one-off event, and here is why:

Firstly, a recap; the ability of air to hold moisture reduces as it cools, and it will then deposit the moisture onto cold surfaces in the form of condensation. For example, in a two-storey house with a floorplan of 100m², there is around 420 cu metres of air, which, at 20 degrees could potentially hold up to 8.4 litres of water vapour. If this warm air passes through the ceiling into the roof space and cools down to 10 degrees, it can then only hold 3.4 litres of water vapour. This means that 5 litres of water will be deposited somewhere if it is not allowed to escape from the roof space. It may not be as dramatic as that in practice, but it illustrates the potential risks.

In many cases, though not all, the problem of condensation appears in newly built or renovated properties. When construction works are completed, the various building materials must dry out and for some, such as concrete, plaster, brickwork and blockwork, there is a lot of water to be removed. To assist in the drying out, the heating is switched on to drive the process. This can result in an overload of water vapour in the air which must be removed. Inevitably, some finds its way through the ceiling into the roof space and then cools, causing condensation to appear on cold surfaces such as the underlay.

Essentially, the ways we can control the risk of condensation build-up in the roofspace are to 1) remove it at source; 2) prevent water vapour from reaching the loft space in the first place; or 3) remove it once it gets there and before it has a chance to build up to harmful levels.

Removing water vapour at source
Simple measures like opening windows during the drying out period can help remove excess water vapour. Kitchens and bathrooms usually have extraction fans, though dehumidifiers can be placed in the worst affected areas during the drying out phase.

Preventing water vapour passing from the living areas
To prevent as much water vapour as possible passing from the living space into the cold roof space, it is necessary to install effective vapour barriers in the ceiling construction. It is difficult, if not impossible, to construct a totally air and vapour-tight ceiling, so British Standard BS 9250 gives guidance on minimising air leakage through junctions and penetrations such as light fittings, loft hatches etc. to create a ‘continuous’ ceiling. Greater thermal efficiency is achieved, and the risk of condensation reduced if we prevent air leakage through the ceiling. This is certainly possible to achieve in new buildings, though it can be more difficult in existing buildings.

Preventing condensation in the roof space
If we can prevent as much water vapour from reaching the roof space as possible, the rest can be taken care of by ventilation of the loft space, or vapour-permeable underlay, or a combination of both.
Vapour-permeable and air-permeable roofing underlays are hugely beneficial in helping to prevent harmful levels of condensation from building up in the roof space. However, it is important to use these products correctly, in accordance with the guidance given in BS 5250 and following the underlay manufacturer’s installation recommendations.

Whether the cause of excess water vapour is due to construction materials drying out, or the lifestyle and number of occupants, the problem is exacerbated during the winter months when the outside air is cold and there is little air movement. Of course, the occupants are less likely to want to open windows in cold weather, which will further impede the dissipation of water vapour from the living spaces. Surprisingly, a problem I often come across is that the occupants, having spotted the condensation in the loft when they go up there to get the Christmas tree, then leave the loft hatch open to try and dry the loft out. This can make the problem worse, as far more water vapour is then freely allowed to pass from the living space into the cold loft space and cool, depositing more condensation.

Building Regulations and BS 5250 recognise that condensation may occur for a period during adverse climatic and internal conditions. It is common to see temporary overloads of condensation appearing on the underlay, which dissipates within a few days with no harm done – usually during very cold but still weather conditions. Though any condensation must not be severe enough to cause damp or staining on internal surfaces or cause damage to the structure generally. If the ceilings, underlay and any recommended additional roof space ventilation are correctly installed, then it is unlikely that condensation within the roof structure will be a regular or harmful problem once the initial drying out phase has passed.

In the unlikely event that condensation persists in a cold roof, it may be necessary to fit extra tile ventilators. The location of the ventilators depends on several factors such as roof geometry and pitch, but as a simple guide, fit some in the tile courses just above the horizontal insulation at each side of the roof to aid crossflow ventilation. Additional ventilators at high level would act to draw air in through the lower ventilators. Only fit extra roof space ventilation as a last resort, as excessive cold air reduces the thermal efficiency of insulation.

It is even more unlikely for excessive condensation to occur in a correctly installed warm roof construction. Venting a completed warm roof would be extremely difficult, if not impossible, therefore the way to solve a condensation problem would be to provide measures to remove water vapour at source, such as additional extractor fans or dehumidifiers.

• Often, excessive condensation accumulating in a correctly installed roof will be a one-time event as building materials dry out. Make sure the building is well ventilated and use dehumidifiers to remove excess water vapour.
• Temporary and occasional condensation events on underlay are not necessarily harmful so long as it quickly dissipates without causing harm to other surfaces such as timber, insulation, or ceilings.
• Follow the manufacturer’s advice when installing permeable underlays and take note of recommendations regarding roof space ventilation.
• Only fit extra roof space ventilation as a last resort, as excessive cold air reduces the thermal efficiency of insulation.

Have you got a pitched roofing query or topic you’d like John to discuss? Tweet @TotContractorUK or @Johnmercer3


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